There is an ongoing need to improve the electrical characteristics of capacitors. In particular, there is a need to lower the ESR of capacitors.
The anode of a typical solid electrolytic capacitor consists of a porous anode body, with a lead wire extending beyond the anode body and connected to the positive mounting termination of the capacitor. The anode is formed by first pressing a valve metal powder into a pellet. Valve metals include Al, Ta, Nb, Ti, Zr, Hf, W, and mixtures, alloys or suboxides of these metals. The anode is sintered to form fused connections between the individual powder particles. There are several resistances to current flow in the anode portion of a solid electrolytic capacitor. The current must flow from the positive mounting termination to the lead wire attached to or embedded in the anode body. Current flows through the portion of the anode lead which extends outside the body of the anode. The current flow through the positive termination and the anode lead produce series resistances which contribute to the equivalent series resistance (ESR) of the finished device. Resistances inside the body of the anode generate parallel resistances which also contribute to the ESR of the finished device. The current travels from the point of lead wire egress to the anode body to all points of the anode body through the path(s) of least resistance. The current must pass from the lead wire into the anode body through points of contact between the lead wire and the particles which make up the porous anode body. The current must then travel through the porous anode body, through small necks of the sintered particles which make up the anode body.
Resistance in the lead wires and in the anodes body is governed by the general equation for resistance.Resistance=resistivity×path length/cross sectional area.
Increasing the cross sectional area available for current flow reduces the resistance as indicated by the equation above. The maximum diameter of the lead wire is determined by the dimensions of the anode. The lead wire diameter can not be greater than the thickness of the anode. Thus the maximum cross sectional area for current to flow through a single cylindrical lead wire is πd2/4 wherein d is the diameter. For a given wire diameter the maximum cross-sectional area for current flow increases proportionately to the number of lead wires connecting the anode body to the positive mounting termination. Thus by increasing the number of wires the resistance in the connection between the positive mounting termination and the anode body is reduced.
Although the lead wire(s) can be attached to the anode body, for example by welding to the top of the body, imbedding the lead wire(s) in the anode body reduces the resistance for current to flow. For lead wires which extend into the porous anode body the cross sectional area available for current to flow from the lead wire to the body is proportional to the external surface area of the lead within the body of the anode. Maximum Area is proportional to π×d×l (for single cylindrical lead wires). The cross sectional area for this resistance term can be increased by increasing the number of lead wires. The path length for current to flow from the lead wire to points of the anode body which are the greatest distance from the lead wire is reduced by utilizing multiple lead wires or non-cylindrical lead wires, for example, flat or ribbon lead wire.
Fluted anodes comprising a furrow or groove on the otherwise monolithic capacitor body as described, for example, in U.S. Pat. Nos. 5,949,639 and 3,345,545 reduce the path length through the internal polymer and increase the cross-sectional area for current to flow through the external polymer. Capacitors utilizing fluted anodes as illustrated in FIG. 1 enjoy much success and this technique is still utilized in current capacitors. The preferred method for connecting a wire to the anode, is to have a wire in place when the compact is pressed. This allows the anode lead to pass through most of the length of the anode compact and maximize contact area between a solid wire anode lead, usually Ta wire for a Ta anode, and the Ta anode. The compact, after removal of binder, is sintered at ca. 1200-1600° C. The area of contact between wire and anode compact is limited by the diameter of the wire which is a function of the thickness of the compact. As the contact area between wire and anode compact decreases, the resistance at the point of contact is increased. As the wire gauge is increased, the internal resistance in the wire is increased. Both increases result in higher equivalent series resistance—ESR—which diminishes the performance of the capacitor.
Even though the use of fluted anodes has enjoyed much success the electrical properties achievable thereby have reached a plateau. Further improvement requires modifications of the anode or the process used to form the anode.
Through diligent research a limitation of radially compressed fluted anodes has been determined to reside in the variation in density created as a function of the process in which these anodes are formed. Density variations have been determined to be dependent on the type of compression. While not restricted to any theory it is postulated that variations in density cause variations in the impregnation of solid electrolyte that serves as the cathode of the electrolytic capacitor. It is also postulated that variations in density cause variations in the external coating coverage of the solid electrolyte. Referring to FIG. 1, the anode, 201, comprising an anode wire, 202, has flutes, 203. The anode is typically formed by radially pressing a powder perpendicular to the anode wire into the desired shape followed by sintering. Single stage compaction with formed punches induces a greater compaction ratio between the flutes than in the non-fluted regions whereby the density between flutes becomes higher than the density in the non-fluted regions. Alternatively, the anode is formed by axially pressing the powder parallel to the anode wire which causes surface burnishing which also inhibits external coating coverage.
By realizing this previously unappreciated phenomenon the present inventors have developed an improved fluted anode, and capacitor, which extends the properties of fluted anodes beyond that previously considered feasible thereby allowing for further improvements in electrical characteristics.
Yet another problem associated with fluted anodes is insufficient conduction between the anode and the cathode lead frame. A scanning electron microscope image of a cross-section of a partial capacitor is shown in FIG. 2. As shown, the cavity of deep, or wide, flutes causes difficulty when attempting to attach the anode over the entire surface of the cathode lead frame. Adequate adhesion is observed in the center of the lead frame with complete silver adhesive coverage of the center section. The two outer sections of the lead frame have inadequate contact seen as small portions of silver between the anode and the cathode lead frame. The loss in contact surface between the lead frame and the anode results in higher resistance at the interface resulting in higher ESR. Longer path lengths also increase ESR. As efforts to further improve the electrical characteristics of capacitors proceeds the ESR limitations caused by the poor surface contact rises to the fore as a limiting parameter in capacitor design. Increasing the conductive path length can also increase ESR and it is a desire to limit path length.
Through diligent research the present inventors have advanced the art of capacitor design, and more importantly the art of anode design and manufacture, beyond that considered feasible in the prior art therefore allowing for further improvements in electrical characteristics in the ongoing pursuit thereof.